This invention relates in general to electrostatography and, more specifically, to a process for fabricating electrostatographic imaging members.
In the art of xerography, a xerographic plate comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive materials exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous dispersions of conductive material in binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles.
When one or more electrically active layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. Curling is undesirable because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, developer applicators and the like during the electrophotographic imaging process thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. A imaging member having a tendency to curl can spontaneously form a roll as small as 3.8 cm in diameter and requires considerable tension to flatten the imaging member against the surface of a separate supporting device. Where the supporting device comprises a large flat area for full frame flash exposure, the imaging member may tear before sufficient flatness can be achieved. Moreover, constant flexing of multilayered photoreceptor belts during cycling can cause stress cracks to form due to fatigue. These cracks print out on the final electrophotographic copy. Premature failure due to fatigue prohibits use of these belts in designs utilizing small roller sizes (e.g. 19 mm or smaller) for effective auto paper stripping. Coatings may be applied to the side of the supporting substrate opposite the electrically active layer or layers to counteract the tendency to curl. However, such coating requires an additional coating step on a side of the substrate opposite from the side where all the other coatings are applied. This additional coating operation normally requires that a substrate web be unrolled an additional time merely to apply the anti-curl layer. Also, many of the solvents utilized to apply the anti-curl layer require additional steps and solvent recovery equipment to minimize solvent pollution of the atmosphere. Further, equipment required to apply the anti-curl coating must be cleaned with solvent and refurbished from time to time. The additional coating operations raise the cost of the photoreceptor, increase manufacturing time, decrease production throughput, and increases the likelihood that the photoreceptor will be damaged by the additional handling. In addition, the anti-curl backing layer can form bubbles during application which requires scrapping of that portion of the photoreceptor containing the bubbles. This in turn reduces total manufacturing yield. Also, difficulties have been encountered with these anti-curl coatings. For example, photoreceptor curl can sometimes still be encountered due to a decrrease in anti-curl layer thickness resulting from wear in as few as 1,500 imaging cycles when the photoreceptor belt is exposed to stressful operating conditions of high temperature and high humidity. The curling of the photoreceptor is inherently caused by internal stress build-up in the electrically active layer or layers of the photoreceptor which promotes dynamic fatigue cracking, thereby shortening the mechanical life of the photoreceptor. Further, the anti-curl coatings occasionally separate from the substrate during extended machine cycling and render the photoconductive imaging member unacceptable for forming quality images. Anti-curl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Moreover, in electrostatographic imaging systems where transparency of the substrate and anti-curl layer are necessary for rear exposure erase to activating electromagnetic radiation, any reduction of transparency due to the presence of an anti-curl layer will cause a reduction in performance of the photoconductive imaging member. Although the reduction in transparency may in some cases be compensated by increasing the intensity of the electromagnetic radiation, such increase is generally undesirable due to the amount of heat generated as well as the greater costs necessary to achieve higher intensity.
Curling of a photoreceptor can be prevented by careful selection of a supporting layer which has a thermal contraction coefficient substantially identical to the thermal contraction coefficient of the charge transport layer. However, such combination limits the choice of materials that can be used for imaging members.